SENSITIVENESS. 69 
reason of the impetus gained when circumstances were 
more favorable. 
Influence of Heat and Moisture on the Stem.—The 
growth of the stem is directly influenced by heat, there 
being in this as in other cases a minimum below which 
growth cannot take place, an optimum at which it takes 
place most vigorously, and a maximum beyond which 
heat is injurious. The favorable influence of heat it is 
which in part overcomes the influence of gravitation, and 
enables the stem to ascend. The stem will grow fastest 
and strongest on the side most exposed to the heat, if 
that heat be not excessive, and this tendency will remove 
it from the soil. Similarly a moist condition of the at- 
mosphere fayors growth, and the stem will grow the fas- 
ter on the side most exposed to the moist vapor, and, 
owing to the convexity so formed, it will in consequence 
bend its free end and its concavity towards the drier side. 
Influence of Contact on Stems—Climbing Plants.— 
The most marked instance of this occurs in the case of 
climbing plants. We have already seen that the young 
growing parts of plants very generally exhibit gyratory 
movements, these movements being produced by inequali- 
ties of growth, now in this direction, now in that, the 
result being that the free end moves round, and that 
these movements are only indirectly affected by tempera- 
ture or light. In the case of climbing plants, such as 
the hop, the dodder, the tendrils of the pea or of the 
vine, which are peculiarly sensitive to contact, these 
movements are much more marked, the object being to 
secure a suitable means of attachment, and so to expose 
the leaves when present to the influence of light and air 
with the least expenditure of force and tissue. Such 
plants, in fact, depend upon others for their mechanical 
support. When the free end of such a plant or a tendril 
